The present invention relates generally to electrochemical cells, especially such cells for wet recovery of metals, so called electrowinning, and cells for chlorine production. In particular it is directed to a clamping device for electrodes of such cells for improved assemblage thereof.
It also relates to an electrode and to an electrochemical cell incorporating such device.
The present invention relates to the technical field of electrochemical processing in general:
However, the present invention is most suitable for electrowinning and certain aspects of chlorine production.
Electrowinning processes are most often of the sulphate or chloride type. The present invention is relevant to both types of process.
Chlorine production processes are of the diaphragm type, the Hg type, or the membrane type. The present invention is relevant to the diaphragm or membrane type of process.
For electrowinning purposes the metal of interest is deposited on the cathode in an electrochemical process, whereby a starter plate or sheet of the metal is used as the cathode. For the anodes various types have been used in the past. Thus, lead (Pb) anodes have been commonly used. The Pb anodes have the shape of a parallelepiped into which a current distributor rail has been moulded, and which is located in parallel with the short sides, thereby acting as a yoke. The yoke rests against current supply rods in the electrolysis vessel. These anodes, when new, exhibit a reasonably good current distribution. but since Pb is consumed in the process there are defined problems and disadvantages, both from an environmental viewpoint, and from a technological. Thus, Pb being a toxic heavy metal renders the waste from the process hazardous and difficult and cumbersome to dispose of. This is especially the case in view of the regulations regarding handling of heavy metals as waste becoming more and more restrictive.
From a technological point of view, the Pb consumption increases the distance between the anode and the cathode, which leads to higher energy consumption and uneven current distribution, and thus to anomalous or erratic deposition of the metal on the cathode, so called dendrites. These electrodes are referred to as consumable electrodes.
Another point of consideration is that the acceptance level of residual Pb in the metal that is produced on the cathode by electrowinning has decreased over the last decades.
Recasting of used Pb anodes is also a controversial activity from an environmental point of view, and will become increasingly restricted.
Titanium anodes for electrowinning, having a grid structure were developed in the 70""s adapted for the cost of electrical energy prevailing at the time. Such grid anodes are composed of parallel wires, where the wire-wire distance is substantially larger than the diameter of the wires. This has the effect of creating an unfavourable current distribution if one attempts to bring anodes and cathodes closer to each other, in the strive for reduced energy consumption. The cathode quality decreases rapidly if a grid anode of this type is allowed to operate in too close proximity of the cathode, namely by promoting development of dendrites of metal. These dendrites may short-circuit the grid, and may even cause physical obstructions leading to damages when anodes are removed from the bath.
Thus, it would be advantageous to have electrodes on which do not promote dendrite formation. A solution to this is to use so called box electrodes.
Such box electrodes in the form of anodes, which are a type of so called dimensionally stable anodes for electrolysis cells (referred to as xe2x80x9cbox anodesxe2x80x9d for reasons set forth below) are well known in the art of electrochemical engineering. See e.g. the patents SE-7407606-8 (Diamond Shamrock Corp.), relating to the basic design of such anodes. and U.S. Pat. No. 3,940,328 (Electronor Corp.) relating to reconstruction of anodes.
In SE-7407606-8 there is disclosed a dimensionally stable anode for use in an electrolysis cell. It comprises a generally cylindrical conductor bar and a pair of metal plates, provided essentially on diametrically opposed sides of said holder. The metal plates are essentially plane parallel and one respective edge portion of each plate is bent to provide a continuous surface, thereby forming a box like structure; hence the designation xe2x80x9cbox anodexe2x80x9d.
However, these box anodes are more expensive than the grid electrodes, and those available today are also difficult to repair because they are assembled by welding. In the repair process often welds have to be broken up, which is very tedious and often lead to substantial scrapping of anodes, further increasing the total cost.
In certain cases the electrochemical separation is carried out with the anode separated from the cathode by enclosing it in a bag of a conventional design. Such bags are soft and flexible, which is required for achieving adequate sealing. The useful life of such a bag is relatively short, and it would be desirable to have access to electrode enclosures with longer useful life. Primarily the resistance against corrosion should be improved. Use of stiffer or harder membranes and diaphragms results in prolonged lifetime, but would require other electrode designs.
From a production point of view, conventional box anodes mostly are manufactured of mesh metal, the surface of which has been levelled out in a rolling operation. Welding and bending operations will become facilitated thereby. However, such conventional box anodes have been developed to function without any enclosures. If such an anode is provided with a membrane or a diaphragm in very close proximity to the anode surface, mass transport will be remarkably deteriorated. Namely, the bubbles forming on the mesh structure will not result in the required circulation. In the preferred design the expanded metal has not been leveled out, which results in the diaphragm/membrane being disposed in close proximity to a very small portion of the total surface. Obviously, these two requirements are contradictory.
Another disadvantage caused by the use of welding as a means of assembling the electrodes, is that there will inevitably be residual stress remaining in the box structure after manufacture. Such residual stresses will have adverse effects of various kinds. The shape of the box may be slightly altered such that it deviates from its rectilinear box shape.
Furthermore, used up anode bags of all kinds present a waste disposal problem in that the material is frequently heavily contaminated with undesirable residual material from the electrolytic processes.
Certain electrochemical processes are operated at high temperature and low pH ( less than 1), and for such conditions the presently available box anodes are vulnerable.
Thus, the present invention seeks to provide means for obtaining an anode structure for electrochemical cells wherein the drawbacks of prior art have been overcome.
In accordance with the invention this object is achieved by refraining from welding the edges of the box anode. Instead the box structure is arrived at by providing a clamping device as claimed in claim 1, an electrode as claimed in claim 7, and an electrochemical cell as claimed in claim 15.
The clamping device of the invention comprises a resilient member cooperating with an abutment member, such that two parallel, essentially flat and rectangular electrode members of an anode assembly, are clamped between one part each of the abutment member and a corresponding part of the resilient member, whereby the clamping means forms a spacer keeping the electrode members in a spaced apart relationship.
The clamping device according to the invention therefore provides for assembly of an electrode of the box anode type without the need of welding, thus reducing or even eliminating residual stress. Disassembly, if necessary, is very easy since there are no welds that need to be broken up. This also has the beneficial effect that the active coating on the anode will not be damaged by the forceful operation needed for breaking up welds.
Preferably the device according to the invention is made of titanium.